Effects Of Urea On The Growth Of Typical Algae In The East China Sea

In this paper, different ratio of nitrate and urea were designed as mix nitrogen sources to culture four typical HABs of the East China Sea, to study the effect of different proportion of organic nitrogen on the growth of phytoplankton, by analyzing growth of cells, specific growth rate, changes of added nutrients, transformation of different forms of nitrogen, particle nitrogen and phosphorus in single cell. Results as follows:(1)After three days of adaptation, the diatoms entered into the exponential growth phase, which was about 3 to 4 days, cell density of diatoms in stationary phase showed a decreasing trend with the increasing content of urea; the adaptation period of dinoflagellates were 5 to 6 days and then dinoflagellates entered into the exponential growth phase, its exponential growth phase was about 7 to 9 days, the dinoflagellates in stationary phase changed little with different content of urea in its growth process.(2)Diatoms and dinoflagellates were different to absorb and utilize urea. During the culture, the diatoms used nitrate priority to urea in the adaptation phase, while the dinoflagellates took advantage of nitrate and urea. After the exponential growth phase, the absorption ratios of the added nitrogen nutrients of the four algae were above 90%.The result showed that the diatoms could absorb urea effectively in the culture.(3)During the culture, with the strengthening of biomass growth and cell activity the content of DON increased with the metabolism. For diatoms, concentration of DON-U reached the maximum (Skeletonema, costatum was 5～6μmol/L. Thalassiosira rotula was 3～4μmol/L) and kept little change in stable stage,which can be interpreted as that the diatoms had a low ability to utilize DON and mainly didn’t absorb DON-U. For the dinoflagellates, the concentration of DON-U reached and could use DON-U.(4)The content of particulate nitrogen in single cell in stationary phase of diatoms increased with increasing content of urea, dinoflagellates varied insignificant with different content of urea, the role of urea on the growth of diatoms and dinoflagellates was different. For the diatoms, urea mainly promoted the growth of cell morphology; for the dinoflagellates, the effect of urea and nitrate were the same and urea was a equivalent nitrogen source to nitrate.(5)Particulate nitrogen and phosphorus in single cell of the four algae in the early stage of incubation increased, it illustrated that diatoms and dinoflagellates could use nitrogen and phosphorus in eutrophication condition. The content of particulate phosphorus in single cell in stationary phase of the four red tide algae showed no significant difference with the varied content of urea which indicating that the varies of content of urea had little effect on the utilization of phosphorus of the four algal cells nutrient.(6)The concentration of urea did not change greatly in a certain range of temperature(17℃～27℃) and pH(7.2～9.2), indicating that the two physical factors had no function on the decomposition of urea.